The present invention relates to a deformable mirror. It is more particularly used as an element of the optical system of a laser beam focusing head.
When power lasers (CO.sub.2, YAG...) are, e.g. used for heat treatment or machining purposes, the incident laser beam is focused onto the material to be treated or machined by a focusing head diagrammatically shown at 1 in FIG. 1. The laser beam 2 is firstly reflected by a reflecting mirror 4, e.g. cooled by water, towards a focusing mirror 6. The function of the latter is to focus the laser beam 2 at a given focal distance onto the target material 8, in accordance with a focal spot and energy distribution which vary according to uses.
For example, in heat treatment, it is of interest to focus the laser beam according to a straight segment and with a constant energy distribution.
To meet with all these possible uses, the operator has to have a range of conventional convergent optics, as well as special optics for modifying the energy distribution. Thus, there is a disturbing and costly lack of flexibility when a wide laser use range is required.
The invention relates to a focusing mirror, whose reflecting surface is deformable. Hitherto deformable mirrors have been of three main types. The first has a reflecting surface formed by a disk bearing on an O-ring and subject to a negative pressure (cf,. e.g. U.S. Pat. No. 4,119,366). This leads to a mirror having a concavity which is difficult to control and which is difficult to stabilize. Moreover, it is virtually impossible to obtain a counter-curvature of the mirror.
The second type of deformable mirror (cf,. e.g. German Pat. No. 3119823) has punctiform actuators fixed to a reflecting disk so as to draw on the same and form fixed or elastic supports permitting the local modification of the shape of the mirror. With such a system it is difficult to have a regular curvature without breaks in the surface because the supports are located and fixed in rigid manner to the disk, which introduces supplementary stresses into the latter. This is even more so when it is a question of obtaining a surface not symmetrical of revolution, e.g. when using the laser in heat treatment.
The third type of deformable mirror has a reflecting surface whose shape is determined by twisting torques (cf,. e.g. U.S. Pat. No. 4,043,644). This solution makes it possible to obtain a regular curvature of the surface in the case when the latter is cylindrical. When said surface is of a random nature, the settings are very difficult to control and the shape obtained is not very stable.